Photodynamic Therapy involves the use of photo-activated chemicals for the treatment of a variety of serious diseases such as cancer. In practice, a photo-activated chemical, e.g. Photofrin by Quadra Logic Technologies, Inc., is injected into the patient, and accumulates in the cancer cells. A period of time later, normally 2 days, laser light is delivered to the cancer site through a diffusing tip of a fiber optic treatment probe to activate the chemical, and treat the cancer.
Typically, laser light is supplied by a medical laser, e.g., a high power argon laser pumping a dye laser head, to the treatment probe through a focusing lens and a coupling fiber (jumper). The light generated by these lasers is in the form of a guassian beam having a small launch angle (output angle), typically 10.degree., and having a wavelength of approximately 630 nanometers (nm).
The treatment probe consists of a length of fiber optic cable with a connector at one end for coupling the light from the laser, or a jumper, to the fiber, and a diffusing tip at the other end through which laser light is diffused and delivered to the patient. There are three types of diffusing tips: cylinder diffusers, sphere diffusers, and microlenses. Each type of diffuser requires a different light power level and time duration for treatment based on calculations by the clinician. The type of diffuser used for treatment is selected following a given protocol based on the type of treatment and the location of the treatment area.
Normally, it is desirable to have a uniform power distribution of laser light leaving the diffuser. The uniform power distribution provides maximum treatment efficiency without damaging (burning) healthy tissue due to areas of high power density. However, because the output angle of the light beam leaving the laser is small, mostly forward projecting lower order modes of the treatment probe fiber are excited during light transmission, and the resulting high density of lower order modes in the diffuser cause an uneven power distribution of the light leaving the diffuser. Another problem caused by the high number of lower order modes is that most of the light projects to the tip of a cylinder diffuser or is focused in a small area of a sphere diffuser or microlens, thereby causing the area of high power density. Hot spots developed in the areas of high power density in the diffuser can cause burning and shorten the useful life of the treatment probe.
The addition of a coupling fiber (jumper) between the laser and the treatment probe tends to mode mix the laser light to some extent so that when the light exits the jumper it has expanded into higher order modes, e.g., 28.degree. full angle. However, treatment fibers normally have more higher order modes available, e.g., 42.degree. full angle, than are filled by the addition of the jumper. In addition, The jumper increases light losses, resulting in reduced power delivered to the diffuser.
There are a number of other known methods for exciting higher order modes of light delivered to a diffuser. A first method is to force a bend in the fiber, thereby causing mode mixing and the resultant filling of higher order modes. However, there is no way to predict which higher order modes are filled, and the stess on the fiber can cause breaks, shortening the life of the fiber.
A second method is to form the treatment probe by butting a diffuser of sapphire against a flat and polished end of a fiber. The light mixes modes as it enters the diffuser resulting in a more uniform power distribution leaving the diffuser. However, there are a number of problems associated with this method. Hot spots are formed at the junction between the diffuser and the fiber. The junction tends to be weaker than the remainder of the probe, and the length of a sapphire cylinder diffuser is limited because of the potential for breakage.
A third method is to taper an end of the fiber into a conical shape and insert that end into a diffuser having a cylindrical shaped aperture. The angle at the end of the fiber induces light loss in a controlled manner, filling higher order modes and resulting in a more even power distribution of light diffused. However, the tip of the fiber still retains low order modes causing high power density and local hot spots. In addition, the length of the probe is limited.
It is therefore a principal object of the present invention to provide an optical fiber structure to control the light modes excited in an optical fiber of a fiber optic treatment probe, thereby eliminating areas of high power density in a light diffuser of the treatment probe, and allowing control of the output light power distribution from the diffuser.
It is another object of the invention to provide a fiber optic probe for direct coupling to a laser light source to eliminate the need for a coupling fiber (jumper) and the associated light losses from the treatment fiber, thereby increasing the light power delivered to the diffuser.
It is a further object of the present invention to provide a fiber optic treatment probe capable of providing a uniform light power distribution from the diffuser.
It is another object of the present invention to provide an improved fiber optic probe that is economical to manufacture and facile in its use.
Other objects of the invention will be in part obvious and in part pointed out in more detail hereinafter
A better understanding of the objects, advantages, features, properties and relations of the invention will be obtained from the following detailed description which sets forth illustrative embodiments indicative of the various ways in which the principles of the invention are employed.